Hollow fiber membrane and method of producing the same

ABSTRACT

A method of producing a hollow fiber membrane includes discharging a polyvinylidene fluoride solution comprising a polyvinylidene fluoride resin and a poor solvent at a temperature above a phase separation temperature into a cooling bath at a temperature below the phase separation temperature to coagulate the polyvinylidene fluoride resin. The hollow fiber membrane comprises a polyvinylidene fluoride resin having spherical structures that have an average diameter in the range of 0.3 to 30 μm.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to hollow fiber membranes andmethods of producing the same. In particular, the present inventionrelates to a method of producing a hollow fiber microfiltration membraneand hollow fiber ultrafiltration membrane to be used in watertreatments, such as drainage treatments, water purification treatments,and industrial water production, and relates to a hollow fiber membraneproduced by the method.

[0003] 2. Description of the Related Art

[0004] Separation membranes such as microfiltration membranes andultrafiltration membranes have been used in various fields such as thefood industry, medical treatment, water production, and waste watertreatment. In recent years, separation membranes have also been used indrinking water production, namely, water purification treatment. In thewater treatment such as water purification, a large volume of water mustbe treated; hence, hollow fiber membranes having a large effectivefiltration area per unit volume are generally used. An improvement inthe water permeability of the hollow fiber membrane allows a reductionin membrane area and a reduction in manufacturing expense due to thereduced size. Such an improvement is also advantageous since exchangingmembranes becomes more cost effective and the membranes require asmaller installation area.

[0005] Fungicides such as sodium hypochlorite are added for sterilizingpermeated water and preventing biofouling of the membrane in some cases.Furthermore, the membranes are washed with acids such as hydrochloricacid, citric acid, and oxalic acid, alkalis such as sodium hydroxide,and surfactants, if necessary. Hence, polyvinylidene fluoride separationmembranes having high chemical resistance have been used recently. Inwater treatment, contamination by chlorine-resistant pathogenicmicroorganisms such as cryptosporidium has become common in the last fewyears. Under such circumstances, hollow fiber membranes must have hightensile properties to prevent contamination by raw water caused byfracture of the membranes. The term “raw water” represents river water,lake water, ground water, seawater, waste water, discharged water, andtreated water thereof.

[0006] Polyvinylidene fluoride separation membranes are prepared by thefollowing methods: (1) A polyvinylidene fluoride solution(polyvinylidene fluoride dissolved in a good solvent) is extruded from aspinneret or cast onto a glass plate held at a temperature that isconsiderably lower than the melting point of the polyvinylidenefluoride, and the shaped resin is brought into contact with a liquidcontaining a nonsolvent to form a porous structure by phase separationinduced by the nonsolvent (wet process disclosed in Japanese ExaminedPatent Application Publication No. 1-22003); and (2) inorganic particlesand an organic liquid are mixed with melted polyvinylidene fluoride, andthe mixture is extruded from a spinneret or molded with a molding pressheld at a temperature that is higher than the melting point of thepolyvinylidene fluoride, the resultant extrudate is solidified bycooling, then the organic liquid and the inorganic particles are removedto form a porous structure (melt extraction process disclosed inJapanese Patent No. 2899903).

[0007] The wet process, however, exhibits unevenness in phase separationin the thickness direction that causes the formation of a membranehaving an asymmetric structure containing macrovoids; hence, themembrane has insufficient mechanical strength. Furthermore, there aremany production parameters on which the structure and the properties ofthe membrane depend; the production steps are not controllable andreproducible. The melt extraction process yields a relatively uniform,high-strength membrane with no macrovoids; however, poor dispersion ofthe inorganic particles can cause defects such as pinholes. Furthermore,the melt extraction process has a disadvantage of extremely highproduction cost.

SUMMARY OF THE INVENTION

[0008] An object of the present invention is to provide a hollow fibermembrane that is composed of a polyvinylidene fluoride resin having highchemical resistance and shows high mechanical strength and high waterpermeability.

[0009] Another object of the present invention is to provide a method ofproducing the hollow fiber membrane with reduced environmental load atlow cost.

[0010] According to an aspect of the present invention, a method ofproducing a hollow fiber membrane includes discharging a polyvinylidenefluoride solution comprising a polyvinylidene fluoride resin and a poorsolvent at a temperature above a phase separation temperature into acooling bath at a temperature below the phase separation temperature tocoagulate the polyvinylidene fluoride resin.

[0011] According to another aspect of the present invention, a hollowfiber membrane comprises a polyvinylidene fluoride resin havingspherical structures that have an average diameter in the range of 0.3to 30 μm.

[0012] According to another aspect of the present invention, a hollowfiber membrane module includes the above hollow fiber membrane.

[0013] According to another aspect of the present invention, a waterseparator includes the hollow fiber membrane module.

[0014] According to another aspect of the present invention, a method ofproducing permeated water from raw water uses the above water separator.

[0015] According to another aspect of the present invention, in a methodof producing permeated water from raw water using a membrane comprisinga polyvinylidene fluoride resin, the method comprises bringing themembrane into contact with chlorine in an amount corresponding to theorganic content (natural organic matter content) in the raw water.

BRIEF DESCRIPTION OF THE DRAWINGS

[0016]FIG. 1 is a phase diagram illustrating typical liquid-liquid phaseseparation;

[0017]FIG. 2 is a phase diagram illustrating typical solid-liquid phaseseparation;

[0018]FIG. 3 is a thermogram of a polymer solution heated at a heatingrate of 10° C./min to a dissolution temperature, held at the dissolutiontemperature for 5 minutes, and cooled at a cooling rate of 10° C./min ina differential scanning calorimeter;

[0019]FIG. 4 is an electron micrograph of a cross-section of a hollowfiber membrane according to the present invention; and

[0020]FIG. 5 is a schematic diagram of a membrane separation apparatus.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0021] Polyvinylidene fluoride resins in the present invention representresins containing vinylidene fluoride homopolymer and/or vinylidenefluoride copolymer. The polyvinylidene fluoride resins may containdifferent types of vinylidene fluoride copolymer. The vinylidenefluoride copolymer has a vinylidene fluoride structural unit. Typicalvinylidene fluoride copolymers are polymers of vinylidene fluoridemonomer and fluorine-containing comonomers, such as vinyl fluoride,tetrafluoroethylene, hexafluoropropylene, and trifluorochloroethylene.These comonomers may be used alone or in combination. The vinylidenefluoride copolymer in the present invention may contain any othermonomer such as ethylene as long as the copolymer exhibits theadvantages in the present invention.

[0022] Poor solvents in the present invention represent liquids thatcannot dissolve 5 percent by weight or more of polyvinylidene fluorideresin at a low temperature of less than 60° C. and can dissolve theresin at a high temperature in the range of 60° C. to the melting pointof the resin (for example, about 178° C. for a vinylidene fluoridehomopolymer resin). In the present invention, good solvents representliquids that can dissolve 5 percent by weight or more of polyvinylidenefluoride resin at a low temperature of less than 60° C., and nonsolventsrepresent liquids that can neither dissolve nor swell the polyvinylidenefluoride resin at any temperature lower than the melting point of thepolyvinylidene fluoride resin. Examples of poor solvents includemedium-chain alkyl ketones, esters, glycol esters and organiccarbonates, i.e., cyclohexanone, isophorone, γ-butylactone, methylisoamyl ketone, dimethyl phthalate, propylene glycol methyl ether,propylene carbonate, diacetone alcohol, and glycerol triacetate. Amongthese, cyclohexanone, isophorone, γ-butylactone, and dimethyl phthalateare preferable. Cyclohexanone and γ-butylactone, are more preferable.Examples of good solvents include lower alkyl ketones, esters, andamides, i.e., N-methyl-2-pyrrolidone, dimethyl sulfoxide, dimethylacetamide, dimethyl formamide, methyl ethyl ketone, acetone,tetrahydrofuran, tetramethylurea, and trimethyl phosphate. Examples ofnonsolvents include water; aliphatic, aromatic, and chlorinatedhydrocarbons, i.e., hexane, pentane, benzene, toluene, methanol, carbontetrachloride, and o-dichlorobenzene, trichloroethylene;hydroxyl-containing liquids, i.e., ethanol and low-molecular weightpolyethylene glycol; and other chlorinated organic liquids.

[0023] In the present invention, the polyvinylidene fluoride resin isdissolved into a poor solvent at a temperature that is higher than aphase separation temperature, namely, 80° C. to 175° C., and preferably100° C. to 170° C., to prepare a polyvinylidene fluoride resin solution.The weight of the polyvinylidene fluoride resin used is in the range of20 to 60 percent by weight, and preferably 30 to 50 percent by weight.The tensile properties of the resulting hollow fiber membrane increasewith the resin concentration; however, an excess resin content resultsin low porosity and thus low water permeability of the hollow fibermembrane. Furthermore, the viscosity of the polymer solution must be ina suitable range in order to prepare hollow fibers. For preparation ofthe polymer solution, different types of poor solvent may be used. Thepoor solvent may contain a good solvent, a nonsolvent, a nucleatingagent, an antioxidant, a plasticizer, a molding aid, and a lubricant, aslong as the polymer solubility does not change substantially. Themixture is agitated at an elevated temperature to prepare a polymerstock solution.

[0024] Meanwhile, in conventional wet processes, the polymerconcentration is within the range of about 10 to 20 percent by weightfor ensuring water permeability. No membrane having high tensileproperties is obtainable from this range. In contrast, the above highpolymer concentration in the present invention enables the hollow fibermembrane to have high tensile properties. In the present invention, thepolymer solution is cooled from a temperature above the phase separationtemperature in the range of 80° C. to 175° C. by cooling liquid or thelike so that the polymer is coagulated. In this process, microsphericstructures connect to each other to form a hollow fiber membrane havingpores. The microspheric structure is assumed to be spherulitic.Spherulites in this process are formed by spherical porous precipitatesof the polyvinylidene fluoride resin from the polyvinylidene fluoridesolution by phase separation. A hollow fiber membrane prepared by thisprocess has higher mechanical strength and water permeability than thathaving a network structure obtained by any conventional wet process.

[0025] Since a high-concentration polymer solution in a poor solventexhibits a large change in viscosity with temperature, the formation ofa hollow fiber membrane from the solution is generally difficult. If theviscosity of the polymer solution is significantly low, the polymercomponent cannot continuously coagulate in a drying unit or a coolingbath and no hollow fiber membrane is obtainable. If the viscosity issignificantly high, the polymer solution is not smoothly discharged fromthe spinneret and no hollow fiber membrane is obtainable. In the presentinvention, the spherical structure is controlled by a combination of aspecific temperature range of the polymer solution and a coolingprocess. Specifically, (1) if the temperature of the polymer solution issignificantly low, gelation or solidification occurs before thedevelopment of spherical structures and thus no porous structure havingwater permeability is formed; and (2) if the temperature issignificantly high, long time is required for cooling, gelation, andsolidification. In this case, large spherical structures are formed andpolymer molecule aggregates that bond these spherical structuresdecrease. Thus, the membrane structure exhibits low mechanical strength.The present invention is accomplished based on these results. Theprinciple of the present invention will now be described in detail.

[0026] Phase separation processes for producing porous membranes arecategorized into a nonsolvent-induced phase separation process thatinduces phase separation by contact with the nonsolvent and athermally-induced phase separation process that induces phase separationby a change in temperature. The thermally-induced phase separationprocess primarily utilizes one of the following two separationmechanisms; liquid-liquid phase separation and solid-liquid phaseseparation. In the liquid-liquid phase separation, a homogeneous polymersolution at a high temperature is separated into a concentrated polymerphase and a diluted polymer phase by a decrease in solubility during acooling step. In the solid-liquid phase separation, a homogeneouspolymer solution at a high temperature is separated into a solid polymerphase formed by crystallization of the polymer and a diluted polymersolution phase during a cooling step (Journal of Membrane Science 117(1996), pp. 1-31). The mechanism is determined by the phase state of thepolymer solution.

[0027]FIG. 1 is a phase diagram of a typical liquid-liquid phaseseparation. In the present invention, the melting point Tm (° C.) andthe crystallization temperature Tc (° C.) of the stock solution aredetermined at a heating/cooling rate of 10° C./min by differentialscanning calorimetry (DSC), unless otherwise specified. A binodal curveis obtained by plotting the phase separation temperatures that aredetermined by measuring clouding points. In the liquid-liquid phaseseparation, the binodal curve lies at a higher-temperature side than thecrystallization curve. The polymer solution is gradually cooled from themelting point. When the polymer solution reaches any temperature on thebinodal curve, the solution is separated into a concentrated polymerphase and a diluted polymer phase. The phase separation continues untilthe solution reaches the crystallization temperature. The final porousstructure after removing the solvent is a matrix structure (sea-islandstructure), although the structure depends on the composition of thepolymer solution and the cooling rate.

[0028]FIG. 2 is a phase diagram of a typical solid-liquid phaseseparation. In this mode, the crystallization curve lies at ahigher-temperature side than the binodal curve. The polymer solution isgradually cooled from the melting point. When the polymer solutionreaches any temperature on the crystallization curve, crystallization ofthe polymer occurs. During a further cooling step, the polymer crystalsgrow. The final porous structure after removing the solvent is aspherulite structure, although the structure depends on the compositionof the polymer solution and the cooling rate.

[0029] For example, any polyvinylidene fluoride/poor solvent systemcauses the solid-liquid phase separation. In the phase diagram of thissystem, the binodal curve lies below the crystallization curve and isnot observed. The relative position of the binodal curve shifts towardsthe high-temperature side as the affinity of the solvent to the polymerdecreases. In this system, no solvent showing liquid-liquid phaseseparation is known.

[0030] In the present invention, the crystallization temperature Tc isdefined as follows: A mixture of a polyvinylidene fluoride resin and asolvent, the mixture having the same composition as that of a polymerstock solution for producing a membrane, is sealed into a DSC cell. TheDSC cell is heated at a heating rate of 10° C./min to a dissolutiontemperature in a DSC apparatus, is held at the dissolution temperaturefor 5 minutes, and is cooled at a cooling temperature of 10° C./min. Therising temperature of the crystallization peak of the DSC curve in thecooling stage is defined as the crystallization temperature Tc (see FIG.3).

[0031] The inventors have found that the crystallization temperature ofthe polymer solution is highly related to the membrane structure formedby the thermally induced phase separation. The present invention ischaracterized in that the crystallization temperature Tc of the polymersolution is in the range of 40° C. to 120° C. In other words, theconditions for forming the membrane are controlled so that thecrystallization temperature becomes higher. Thus, the membranestructure, namely, the spherulite size can be miniaturized. A membranehaving a fine structure exhibits high separability. Conditions affectingthe crystallization temperature of the stock solution are, for example,the polymer concentration, types of polymer (molecular weight, shape ofthe branch, type of copolymer), the type of solvent, and additivesaffecting the crystallization. For example, with increasing polymerconcentration, the crystallization temperature Tc increases while thespherulite size decreases. The inventors have also found that thespherulite size decreases as the crystallization temperature Tcincreases. Furthermore, the inventors have found that the spherulitesize increases as the molecular weight of the polymer increases. Thecrystallization temperature Tc is less correlated with the molecularweight, but is affected by the type of polymer (homopolymer orcopolymer) and the shape of the branch. With substantially the samemolecular weight, the spherulite size tends to decrease when a polymersolution having a higher crystallization temperature Tc, which isdetermined by the shape of the branch and the type of copolymer, isused.

[0032] In the present invention, the type of polymer is preferablyselected so as to increase the polymer concentration and thecrystallization temperature Tc of the polymer solution. Furthermore, thepolymer stock solution preferably contains additives that can shift thecrystallization temperature Tc of the stock solution, such as organicand inorganic salts.

[0033] The results of X-ray diffractometry show the formation of thespherulite structure. The formation of the spherulites is an exothermicreaction. In general, crystals that are first formed during thecrystallization of polymers such as polyvinylidene fluoride resin arecalled primary nuclei. The primary nuclei grow into spherulites. If theformation rate of the primary nuclei is low, heat generated in thegrowth of the primary nuclei inhibits further formation of primarynuclei and facilitates further growth of the generated primary nuclei.The crystal growth continues until the spherulites collide with eachother. Since the crystal growth is terminated by collision, the finalspherulite size depends on the number of the primary nuclei generatedfirst. In a polymer solution having a high crystallization temperatureTc, the crystallization readily proceeds, and the resulting spherulitesize is reduced by the formation of many primary nuclei. In contrast, ina polymer solution having a low crystallization temperature Tc, thecrystallization is inhibited, and the resulting spherulite sizeincreases by the formation of relatively small primary nuclei.

[0034] The crystallization temperature Tc of the polymer solution ispreferably in the range of 40° C. to 120° C., more preferably 45° C. to105° C., and most preferably 48° C. to 95° C. A crystallizationtemperature Tc of less than 40° C. does not cause the formation of afine membrane structure. A crystallization temperature exceeding 120° C.causes crystallization of the polymer in the polymer solution; hence,equipment for forming the membrane, such as a dissolver and pipes, mustbe controlled at high temperatures, resulting in energy loss.Furthermore, the solution must be rapidly cooled from a high temperatureto a crystallization temperature. In addition, the polymer concentrationmust be high otherwise a high-porosity membrane cannot be obtained.

[0035] As described above, the polyvinylidene fluoride resinconcentration in the polymer solution is preferably in the range of 20to 60 percent by weight and more preferably 30 to 50 percent by weight,in view of compatibility between the mechanical strength and waterpermeability of the hollow fiber membrane, and formability into thehollow fiber membrane. A polymer concentration of less than 20 percentby weight causes a decrease in the crystallization temperature Tc of thepolymer solution. Such a decrease inhibits the formation of a finemembrane structure. A polyvinylidene fluoride resin concentration in therange of 30 to 60 percent by weight facilitates production of a membranehaving high permeability and a fine membrane structure.

[0036] The weight average molecular weight of the polyvinylidenefluoride resin in the stock solution is preferably at least 2×10 ⁵. Aweight average molecular weight of less than 2×10 ⁵ leads to lowviscosity of the solution that impairs formability of the membrane and adecrease in the mechanical strength of the membrane. A polymer having ahigh molecular weight causes an increase in viscosity of the solution,which inhibits the crystal growth. As a result, many spherulite nuclei,which are beneficial in the formation of a fine structure, are formed.More preferably, the weight average molecular weight of thepolyvinylidene fluoride resin is in the range of 3×10⁵ to 3×10⁶.

[0037] In the present invention, the polymer solution is discharged froma double pipe spinneret to form a hollow fiber membrane, and the hollowfiber membrane is cooled to obtain a gel product. The spinnerettemperature Ts (° C.) in the present invention represents thetemperature of the bottom surface of the spinneret that discharges thepolymer solution. In the present invention, the spinneret temperature Tsis controlled so as to satisfy the relationship Tc≦Ts≦Tc+90, preferablyTc+10≦Ts≦Tc+85, and more preferably Tc+20≦Ts≦Tc+80. The crystallizationtemperature Tc is preferably as low as possible for cooling efficiency;however, an excessively low crystallization temperature Tc impairs theformability of the membrane. If the spinneret temperature Ts is lowerthan the crystallization temperature Tc, the polymer is crystallized atthe spinneret and cannot be satisfactorily discharged. If the spinnerettemperature Ts is larger than the crystallization temperature Tc+90° C.,the resulting membrane retaining heat is insufficiently cooled duringthe cooling step and a fine membrane structure is not obtained. Forexample, the polymer solution is discharged through a double pipespinneret for spinning a hollow fiber membrane, and the spun hollowfiber membrane is introduced to a drying section having a predeterminedlength and to a cooling bath to coagulate the hollow fiber membrane.Before the polymer solution is discharged from the spinneret, thepolymer solution is preferably filtered through a 5 to 100 μmstainless-steel filter. The dimensions of the spinneret are determinedin view of the size and structure of the hollow fiber membrane.Preferably, the spinneret has a slit outer diameter of 0.7 to 10 mm, aslit inner diameter of 0.5 to 4 mm, and an injection pipe of 0.25 to 2mm. The spinning draft (the drawing rate to the linear discharged rateof the stock solution at the spinneret) is preferably in the range of0.8 to 100, more preferably 0.9 to 50, and most preferably 1 to 30, andthe distance between the spinneret surface and the cooling bath surfaceis preferably in the range of 10 to 1,000 mm. The spinneret temperatureTs may be different from the dissolution temperature. Preferably, thedissolution temperature is higher than the spinneret temperature Ts forrapidly completing uniform dissolution. The hollow fiber polymer iscoagulated into a hollow fiber membrane, as described above. Preferably,the coagulation bath containing a poor solvent has a temperature in therange of 0° C. to 50° C. and more preferably 5° C. to 30° C. and a poorsolvent concentration in the range of 60 to 100 percent by weight andmore preferably 75 to 90 percent by weight. The coagulation bath maycontain two or more poor solvents in combination. Furthermore, thecoagulation bath may contain any good solvent and nonsolvent within theabove poor solvent concentration. Rapid cooling with a large temperaturedifference between the polymer solution temperature and the polymerdissolution temperature facilitates the formation of fine spherulitestructures that are bonded by the coagulated polymer, forming a membranestructure having high permeability and high tensile properties. A poorsolvent contained in the cooling bath at a considerably highconcentration suppresses nonsolvent-induced phase separation, and theresulting hollow fiber membrane does not have a dense layer on thesurface. If the cooling bath contains a high concentration of nonsolventsuch as water, the resulting membrane has a dense surface layer and doesnot exhibit water permeability even after the membrane is stretched.

[0038] In general, for the formation of the hollow sections in thehollow fiber membrane, the polymer solution is discharged while gas orliquid is being supplied into the hollow section of the inner tube ofthe spinneret. In the present invention, a hollow section-forming liquidcontaining 60 to 100 percent by weight of a poor solvent is preferablysupplied. In the hollow section-forming liquid, the content of the poorsolvent is more preferably in the range of 70 to 100 percent by weightand most preferably 80 to 100 percent by weight. The liquid containing ahigh amount of poor solvent suppresses nonsolvent-induced phaseseparation and facilitates the formation of fine spherical structures.Different poor solvents may be used in combination. The liquid maycontain small amounts of a good solvent and/or nonsolvent within theabove range.

[0039] The cooling bath and the hollow section-forming liquid may be-thesame or different, and may be appropriately selected according to thetarget properties of the hollow fiber membrane. If the same poor solventis used in the polymer solution, the cooling bath, and the hollowsection-forming liquid, the poor solvent can be easily recovered. Anyvessel may be used for containing the cooling bath. The cooling bath maybe circulated or renewed while the composition and temperature are beingcontrolled. Alternatively, a cooling liquid may be circulated in a pipein which the hollow fiber membrane travels, or may be sprayed onto thehollow fiber membrane that travels through air.

[0040] In the present invention, the polymer solution is cooled at anaverage cooling rate Vt in the range of 2×10³° C./min to 10⁶° C./minwhen the polymer solution is cooled to the crystallization temperatureTc. The average cooling rate Vt is preferably in the range of 5×10³°C./min to 6×10⁵° C./min and more preferably 10⁴° C./min to 3×10⁵°C./min. As a result of phase separation at the above average coolingrate Vt, the hollow fiber membrane has a finer structure. The averagecooling rate Vt during the formation of the membrane in the presentinvention is determined by either of the following methods (a) and (b):Case (a): the temperature of the cooled polymer solution reaches thecrystallization temperature Tc in air.

Vt=(Ts−Tc)/t(sc)

[0041] wherein Ts is the temperature (° C.) of the spinneret, Tc is thecrystallization temperature (° C.), and t(sc) is the elapsed time fromdischarging the stock solution to reaching the crystallizationtemperature Tc.

[0042] For the determination of the elapsed time t(sc), the time toreach the crystallization temperature Tc in air can be measured, forexample, by thermography, and the elapsed time t(sc) is calculated fromthe distance from the spinneret to a point when the solution reaches thecrystallization temperature Tc and the spinning rate. Case (b): thetemperature of the cooled polymer solution reaches the crystallizationtemperature Tc in the cooling bath.

Vt=(Ts−Ta)/t(sa)

[0043] wherein Ts is the temperature (° C.) of the spinneret, Ta is thetemperature (° C.) of the cooling bath, and t(sa) is the elapsed timefrom discharging the stock solution to reaching the temperature of thecooling bath.

[0044] In the measurement of the elapsed time t(sa), the polymersolution is assumed to reach the temperature of the cooling bathimmediately after the polymer solution is dipped into the cooling bath.Thus, the elapsed time t(sa) can be calculated from the distance fromthe spinneret to the cooling bath and the forming rate of the membrane.

[0045] An average cooling rate of less than 2×10³° C./min inevitablycauses the formation of large structures that do not show satisfactorypermeability. An average cooling rate exceeding 10⁶° C./min requires asignificantly high cooling rate. When the polymer solution is cooled atsuch a high cooling rate in the cooling bath, discharge and cooling areunstable and the resulting membrane does not always show satisfactoryproperties.

[0046] The reason for a fine structure being obtained by a high coolingrate when the temperature of the cooled polymer solution reaches thecrystallization temperature Tc is as follows: Heat generated by theformation of primary nuclei during the cooling step is removed by rapidcooling; crystal growth is inhibited and many primary nuclei suitablefor forming fine structures are simultaneously formed.

[0047] The finely porous membrane obtained by the above method has astructure of fine bonded spherulites and pores therebetween. Thismembrane has higher mechanical strength, higher water permeability, andhigher separability than conventional fine porous membranes.

[0048] The cooled gel membrane is dipped into an extraction solvent oris dried to remove the solvent from the membrane. A porous membrane isthereby prepared. The porous membrane may be drawn to increase porosityand to decrease the pore diameter due to elongation or tearing at theinterfaces between the spherulites, and to enhance the mechanicalstrength due to orientation of the membrane. For obtaining a hollowfiber membrane having higher water permeability, the drawing temperatureis preferably in the range of 50° C. to 140° C., more preferably 55° C.to 120° C., and most preferably 60° C. to 100° C., while the drawingratio is preferably in the range of 1.1 to 5 times, more preferably 1.1to 4 times, and most preferably 1.1 to 3 times. The porous membranecannot be uniformly drawn at a temperature below 50° C. and will bestructurally damaged at weak portions. However, parts of the sphericalstructures and the polymer molecules connecting the spherical structuresare uniformly drawn at a temperature in the range of 50° C. to 140° C.As a result, many fine long pores having high stretch properties andwater permeability are formed. If the membrane is drawn at a temperatureexceeding 140° C., which is near the melting point of the polyvinylidenefluoride resin, the spherical structures are melted and the formation offine pores is inhibited. Thus, the water permeability is not improved.Preferably, drawing is performed in a liquid because of ease oftemperature control; however, drawing may be performed in gas such assteam. The liquid is preferably water. In drawing at a temperature of90° C. or more, low-molecular weight polyethylene glycol may be usedinstead of water. Alternatively, drawing may be performed in a mixtureof different liquids, for example, water and polyethylene glycol.

[0049] If such drawing is not employed, water permeability is low butfiltration properties are improved compared with that of the drawnmembrane. Thus, drawing may be employed depending on the desiredapplication of the hollow fiber membrane.

[0050]FIG. 4 is an electron micrograph of a cross-section of the hollowfiber membrane according to the present invention.

[0051] The hollow fiber membrane has spherical structures having anaverage diameter in the range of 0.3 to 30 μm, preferably 0.5 to 20 μm,and more preferably 0.8 to 10 μm. In particular, it is preferable thatthe interior of the hollow fiber membrane has spherical structures. Inthe interior, the spherical structures are bonded and have porestherebetween. Thus, the mechanical strength and water permeability arehigher than those of conventional network structures. Here, the interiorincludes the substantial inner portion and/or the inner surface of thehollow fiber membrane but excludes the outer surface. The diameter ofthe spherical structures is determined by averaging the diameters of atleast 10, and preferably at least 20 spherical structures selected atrandom in a scanning electron micrograph at a magnification that canclearly observe a cross section and/or an inner surface of the hollowfiber membrane. Preferably, the photograph may be analyzed using animage analyzer to determine equivalent circular diameters of the images.The density of the spherical structure is preferably in the range of 10³to 10⁸ /mm² and more preferably 10⁴ to 10⁶/mm². The density isdetermined by counting the number of spherical structures in a unit areain the micrograph. The spherical structures are substantially sphericalor oval, and the circularity (short diameter/long diameter) ispreferably at least 0.5, more preferably at least 0.6, and mostpreferably at least 0.7.

[0052] Preferably, the hollow fiber membrane according to the presentinvention has fine pores having an average diameter in the range of 0.01to 20 μm, more preferably 0.01 to 10 μm, and most preferably 0.01 to 5μm in the outer surface. The pores in the outer surface may have anysuitable shape. The average of the equivalent circular diameters ofthese pores is preferably determined from the photograph using an imageanalyzer. Alternatively, the average of the equivalent circulardiameters may be determined by averaging the short diameter and longdiameter averages of the observed pores.

[0053] The spherical structure is preferably observed at the innerportion of a cut section of the hollow fiber membrane.

[0054] The outer diameter and the thickness of the hollow fiber membranemay be determined depending on the target volume of the permeable waterin a membrane module, in view of the pressure loss in the longitudinaldirection inside the hollow fiber membrane, as long as the hollow fibermembrane has predetermined mechanical strength. A larger outer diameteris advantageous for pressure loss but disadvantageous for the membranearea due to a reduction in the number of packed hollow fiber membranes.In contrast, a smaller outer diameter is advantageous for the membranearea due to an increase in the number of packed membranes butdisadvantageous for pressure loss. A smaller thickness is preferable aslong as the mechanical strength is maintained. Accordingly, the outerdiameter of the hollow fiber membrane is preferably in the range of 0.3to 3 mm, more preferably 0.4 to 2.5 mm, and most preferably 0.5 to 2 mm.The thickness is preferably 0.08 to 0.4 times, more preferably 0.1 to0.35 times, and most preferably 0.12 to 0.3 times the outer diameter.

[0055] It is preferable that the hollow fiber membrane of the presentinvention does not substantially have macrovoids. Here, “macrovoids”represent voids having a diameter of 50 μm or more. In the presentinvention, the number of macrovoids is preferably 10/mm² and morepreferably 5/mm² and most preferably zero.

[0056] Preferably, the hollow fiber membrane of the present inventionhas a water permeability in the range of 0.1 to 10 m³/m²·hr, morepreferably 0.5 to 9 m /m hr, and most preferably 1 to 8 m³/m²·hr at 100kPa and 25° C., has a tensile strength in the range of 0.3 to 3 kg perfiber, more preferably 0.4 to 2.5 kg per fiber, and most preferably 0.5to 2 kg per fiber, and has an elongation at break in the range of 20% to1,000%, more preferably 40% to 800%, and most preferably 60% to 500%. Ahollow fiber membrane satisfying these ranges exhibits high waterpermeability without being damaged under usual operating conditions.

[0057] In the hollow fiber membrane of the present invention, thepolyvinylidene fluoride main chain preferably has hydrophilic functionalgroups. The hydrophobic polyvinylidene fluoride resin easily trapscontaminants in water, resulting in decreased water permeability.Furthermore, the trapped contaminants cannot be easily removed bywashing. Introduction of the hydrophilic groups prevents the trapping ofthe contaminants and facilitates their removal by washing. As a result,the filtration membrane has a prolonged operation life. Examples ofhydrophilic groups are hydroxyl, amino, and carboxyl. These hydrophilicgroups may be introduced alone or in combination. Since introduction ofa large number of hydrophilic groups decreases the mechanical strengthof the hollow fiber membrane, a small number that cannot be determinedby general analytical methods is introduced on the inner and outersurfaces and the surfaces of the porous structures of the hollow fibermembrane. However, the introduction of the hydrophilic groups can beevaluated by an increase in the water penetration rate.

[0058] The hydrophilic functional groups can be introduced by any knownprocess. Examples of methods for introducing hydroxyl groups are areaction of a polyoxyalkylene having hydroxyl end groups in the presenceof base disclosed in Japanese Unexamined Patent Application PublicationNo. 53-80378; a chemical treatment in a strong alkaline solutioncontaining an oxidizing agent disclosed in Japanese Unexamined PatentApplication Publication No. 63-172745; and grafting of a monomercontaining a neutral hydroxyl group disclosed in Japanese UnexaminedPatent Application Publication No. 62-258711. A more preferred method inthe present invention is dehydrofluorination of a hollow fiber membranein an aqueous alkaline solution and then treatment of the membrane in anaqueous solution containing an oxidizing agent. This method has anadvantage in that the process can be performed in a diluted alkalinesolution and a diluted oxidizing agent solution, whereas the methoddisclosed in Japanese Unexamined Patent Application Publication No.63-172745, which uses an oxidizing agent in the presence of strongalkaline, requires a large amount of strong oxidizing agent, i.e.,permanganate or bichromate, and treatment of waste water containingheavy metal ions, although hydroxyl groups are infallibly introduced.Specifically, the dehydrofluorination method can be achieved in a 0.001-to 1-N aqueous alkaline solution in combination with hydrogen oxide orhypochlorite as the oxidizing agent. Examples of usable alkalis areinorganic hydroxides, i.e. sodium hydroxide and potassium hydroxide, andtertiary amines such as triethylamine. Alternatively, the hollow fibermembrane may be treated with an alkali followed by oxidation inozone-containing water, as is disclosed in Japanese Unexamined PatentApplication Publication No. 5-317663.

[0059] Examples of reactions for introducing amino groups are reactionof compounds containing primary or secondary amino groups disclosed inJapanese Unexamined Patent Application Publication Nos. 59-169512 and1-224002.

[0060] An example of reactions for introducing carboxy groups isgrafting a carboxyl-containing monomer.

[0061] Preferably, the hollow fiber membrane is immersed into an alcoholor aqueous alcohol before the introduction of the hydrophilic functionalgroups in order to introduce these groups homogeneously. Examples ofalcohols are methanol, ethanol, 1-propanol, 2-propanol, 1-butanol,2-butanol, isobutyl alcohol, and t-butyl alcohol. The alcohol content inthe aqueous alcohol is preferably at least 10 percent by weight, morepreferably at least 20 percent by weight, and most preferably at least30 percent by weight.

[0062] The hollow fiber membrane produced by the above method may beused in hollow fiber membrane modules that collect permeated water. Onetype of module is a cylindrical container containing a bundle of hollowfiber membranes, an end or two ends of the bundle being fixed with anepoxy resin or the like. Another type of module includes hollow fibermembranes arranged in a flat plate, two ends of the hollow fibermembranes being fixed. The hollow fiber membrane module is generallyprovided with a compression means, i.e., a pump or a difference in waterlevel, at an end for supplying raw water, or a suction means, i.e., apump or siphon at the other end for collecting the permeated water. Thehollow fiber microfiltration membrane is thereby used as a waterseparating apparatus that produces purified permeated water from rawwater by membrane filtration. The term “raw water” represents riverwater, lake water, ground water, seawater, waste water, dischargedwater, and treated water thereof.

[0063] In the method for making permeated water from raw water using themembrane comprising the polyvinylidene fluoride resin, the membrane ispreferably brought into contact with chlorine in an amount correspondingto the organic content in the raw water. The inventors found that thepolyvinylidene fluoride membrane must be brought into contact withchlorine at a prescribed time interval during the filtration operationto ensure normal operation. The inventors also found that the amount ofchlorine in contact with the membrane is closely connected to theorganic content in the supplied raw water. It is known that naturalorganic matter such as fumic substances in raw water function as foulingsubstances for membranes (Water Science and Technology: Water SupplyVol. 1, No. 4, pp. 40-56). Chlorine is believed to prevent trapping ofthe organic matter on the membrane, decompose the trapped organicmatter, and facilitate detachment of the trapped organic matter from themembrane. In particular, the hydrophobic polyvinylidene fluoride resinmembrane easily causes fouling compared with hydrophilic membranes.Thus, the above chlorine treatment is effective for preventing foulingof the membrane. In the membrane having spherical structures accordingto the present invention, fouling will easily occur on the unevensurface and micropores; thus, such chlorine treatment is effective forpreventing fouling. However, an excess amount of chlorine causeseconomic and health problems such as formation of trihalomethanes,although it ensures stable operation. Hence, the chlorine content ispreferably a minimum corresponding to the organic content in the rawwater.

[0064] In the present invention, the organic content in the water may bedetermined by various processes, such as total organic carbon (TOC),chemical oxygen demand (COD), biochemical oxygen demand (BOD), potassiumpermanganate consumption, and UV absorbance at 260 nm. Among these,highly precise and convenient TOC is preferred. The amount of chlorinedosing C (mg/l·min) is 0.01 to 10 times and preferably 0.03 to 5 timesthe TOC (mg/l) in the raw water supplied for each minute. The TOCrepresents an average TOC in the raw water and is determined by astatistical method in view of seasonal and daily variations. Chlorinecan be brought into contact with the membrane by various methods: (1)continuously adding a constant concentration of chlorine to the suppliedraw water; (2) intermittently adding a constant concentration ofchlorine to the supplied raw water; (3) adding a variable concentrationof chlorine in response to a variation in water quality; (4) adding aconstant concentration of chlorine to back washing water so that themembrane is brought into contact with chlorine only during back washingoperations; (5) adding a constant concentration of chlorine to backwashing water for every several back washing operations; and (6) anycombination of methods (1) to (5). Methods (1), (2), (4), a combinationof methods (1) and (4), and a combination of methods (2) and (4) arepreferred because of their simple operation and significant effect ofthe added chlorine. In the intermittent addition and the back washingaddition, the contact amount of chlorine may be an average concentrationwithin a prescribed time. An aqueous sodium hypochlorite, which can behandled easily and is inexpensive, is the most preferable source forgenerating chlorine in the present invention. Calcium hypochlorite,chlorine gas, and liquefied chlorine may also be used.

EXAMPLES

[0065] The present invention will now be described by nonlimitingEXAMPLES.

[0066] The parameters used in the present invention were measured asfollows:

[0067] (1) Melting Point Tm and Crystallization Temperature Tc

[0068] A mixture of a polyvinylidene fluoride resin and a solvent, themixture having the same composition as that of a polymer stock solutionfor producing a membrane, was sealed into a DSC cell. The DSC cell washeated at a heating rate of 10° C./min using a DSC-6200 made by SeikoInstruments Inc. The starting temperature of a melting peak observed inthe heating step was defined as a uniform melting temperature Tm. TheDSC cell was maintained at a dissolution temperature for 5 minutes andwas cooled at a cooling rate of 10° C./min. The rising temperature ofthe crystallization peak observed during the cooling step was defined asthe crystallization temperature Tc (FIG. 3).

[0069] (2) Clouding Point

[0070] The above mixture was sealed with a preparat, a cover glass, andgrease. The specimen was heated to a dissolution temperature and wasmaintained at the temperature for 5 minutes using a cooling and heatingunit LK-600 made by Japan Hightech for microscopes to dissolve thepolyvinylidene fluoride resin. The specimen was cooled at a cooling rateof 10° C./min. The clouding temperature observed during the cooling stepwas defined as the clouding point.

[0071] (3) Average Cooling Rate Vt

[0072] The average cooling rate Vt was calculated using the followingequation according to Case (b) above (the temperature of the cooledpolymer solution reached the crystallization temperature Tc in thecooling bath), unless otherwise specified:

Vt=(Ts−Ta)/(dry distance/extruding rate of polymer solution)

[0073] wherein Ts is the temperature (° C.) of the spinneret, Ta is thetemperature (° C.) of the cooling bath, and the dry distance representsthe distance between the spinneret surface and the cooling bath surface.

[0074] (4) Permeability

[0075] Reverse osmosis membrane treated water at 25° C. was fed intocompact hollow fiber membrane modules (length: about 20 cm, number ofhollow fiber membranes: 1 to 10) by a driving force of differentialpressure corresponding to a 1.5 m difference in water level to measurethe volume of the permeated water for a prescribed time. The volume wasconverted into that for a pressure of 100 kPa.

[0076] (5) Rejection of Polystyrene Latex

[0077] A water composition of reverse osmosis membrane treated water andSeradyn uniform latex particles having a particle size of 0.309 μm wassubjected to cross-flow filtration at a supply pressure of 3 kPa and anaverage linear supply rate of 20 cm/s per area-to obtain permeatedwater. The polystyrene latex concentrations of the supplied water andthe permeated water that was collected 30 minutes after starting thefiltration were determined with a UV-visible light spectrophotometer.The Rejection Rej (t) was determined by the following equation:

Rej=(1−Cb/Ca)×100

[0078] wherein Ca was the polystyrene latex concentration (ppm) in thesupplied water and Cb was that (ppm) in the permeated water.

[0079] (5) Tensile Strength and Elongation of Hollow Fiber Membrane

[0080] Swollen membranes with a length of 50 mm were drawn at acrosshead speed of 50 mm/min under a full-scale weight of 2,000 g usinga tensilometer to determine the tensile strength and elongation at breakof each membrane.

EXAMPLES 1 to 5 AND COMPARATIVE EXAMPLES 1 to 4

[0081] Vinylidene fluoride homopolymer was used as the polymer accordingto the present invention, cyclohexanone was used as the poor solvent,and an aqueous cyclohexanone solution was used as the hollowsection-forming liquid, and the cooling bath. In accordance with Table1, each polymer having a prescribed weight average molecular weight wasdissolved into the poor solvent at a given temperature to prepare apolymer solution having a polymer concentration shown in Table 1. Whilea hollow section-forming liquid containing a prescribed amount of poorsolvent at a prescribed temperature was being discharged into the hollowsection of the inner tube of a spinneret, the polymer solution wasdischarged from the spinneret at a prescribed temperature into a coolingbath containing a prescribed amount of poor solvent and maintained at aprescribed temperature to coagulate the polymer. The properties of theresulting hollow fiber membranes are shown in Table 1. In COMPARATIVEEXAMPLE 1, no hollow fiber membrane was formed because of significantlylow viscosity of the polymer solutions discharged from the spinneret.

[0082] In COMPARATIVE EXAMPLE 2, no hollow fiber membrane was formedbecause of significantly high viscosity of the polymer solutions.

[0083] In COMPARATIVE EXAMPLE 3, the resulting hollow fiber membrane hadno spherical structure and thus did not show permeability. Thepermeability after stretching was at most 0.2 m³/m²·hr at a differentialpressure of 100 kPa and 25° C. Furthermore, the membrane was not able tobe stretched uniformly and was easily broken during the drawing step.

[0084] In COMPARATIVE EXAMPLE 4, the polymer was dissolved for 12 hours,but no uniform solution was obtained. This solution was gelated whenplaced into a hopper of the spinning machine, and no hollow fibermembrane was obtained. TABLE 1 Cooling Cooling Hollow Permeability M.W.of Polymer Dissolving Spinneret bath bath section- Outer Inner (m³/m² ·hr) Tensile Elongation polymer conc. temp. temp. temp. conc. formingliquid diameter diameter at 100 KPa, strength at break (×10⁴) (wt %) (°C.) (° C.) (° C.) (wt %) conc. (wt %) (mm) (mm) 25° C. (g/fiber) (%) EX.1 28.4 55 160 125 30 90 100 1.70 0.99 0.70 540 75 EX. 2 35.8 50 160 12025 85 100 1.63 1.00 0.91 720 68 EX. 3 41.7 40 160 130 20 80 100 1.520.88 1.21 890 59 EX. 4 41.7 20 140  95 10 80  95 1.44 0.92 1.35 420 63EX. 5 57.2 35 170 155 15 80 100 1.51 0.89 0.68 730 58 CE. 1 41.7 18  90 90 — — — — — — — — CE. 2 41.7 62 170 160 — — 100 — — — — — CE. 3 41.740 185 180 30 80 100 — — (0.20) — — CE. 4 28.4 20  78 — — — — — — — — —

EXAMPLE 6

[0085] The hollow fiber membrane prepared in EXAMPLE 1 was drawn to 2.0times in water at 88° C. The stretched hollow fiber membrane had anouter diameter of 1.55 mm, an inner diameter of 0.95 mm, a permeabilityof 1.9 m³m²×hr at a differential pressure of 100 kPa and 25° C., atensile strength of 880 g/fiber, and an elongation at break of 55%.

EXAMPLE 7

[0086] The hollow fiber membrane prepared in EXAMPLE 2 was drawn to 2.5times in polyethylene glycol (molecular weight: 400) at 110° C. Thestretched hollow fiber membrane had an outer diameter of 1.40 mm, aninner diameter of 0.90 mm, a permeability of 2.5 m³/m²×hr at adifferential pressure of 100 kPa and 25° C., a tensile strength of 1,250g/fiber, and an elongation at break of 50%.

EXAMPLE 8

[0087] The hollow fiber membrane prepared in EXAMPLE 3 was drawn to 3.0times in water at 85° C. The stretched hollow fiber membrane had anouter diameter of 1.30 mm, an inner diameter of 0.75 mm, a permeabilityof 3.6 m²/m²×hr at a differential pressure of 100 kPa and 25° C., atensile strength of 1,720 g/fiber, and an elongation at break of 48%.

EXAMPLE 9

[0088] The hollow fiber membrane prepared in EXAMPLE 4 was drawn to 3.5times in water at 85° C. The stretched hollow fiber membrane had anouter diameter of 1.20 mm, an inner diameter of 0.70 mm, a permeability.of 4.8 m³/m²×hr at a differential pressure of 100 kPa and 25° C., atensile strength of 610 g/fiber, and an elongation at break of 50%.

EXAMPLE 10

[0089] The hollow fiber membrane prepared in EXAMPLE 5 was drawn to 4.0times in water at 85° C. The stretched hollow fiber membrane had anouter diameter of 1.35 mm, an inner diameter of 0.80 mm, a permeabilityof 2.1 m³m²×hr at a differential pressure of 100 kPa and 25° C., atensile strength of 1,380 g/fiber, and an elongation at break of 45%.

EXAMPLE 11

[0090] Into 60 percent by weight of cyclohexane, 30 percent by weight ofpolyvinylidene fluoride homopolymer having a weight average molecularweight of 358,000 and 10 percent by weight oftetrafluoroethylene/vinylidene fluoride copolymer were dissolved at 165°C. The polymer solution was discharged from a spinneret at 145° C. while100% cyclohexanone (hollow section-forming liquid) was being dischargedinto the hollow section of the inner tube of the spinneret, and wassolidified in a cooling bath containing 90 percent by weight ofcyclohexanone at 30° C. The fiber was drawn to 3.0 times in water at 80°C. The stretched hollow fiber membrane had an outer diameter of 1.40 mm,an inner diameter of 0.90 mm, a permeability of 1.5 m³/m²×hr at adifferential pressure of 100 kPa and 25° C., a tensile strength of 1,580g/fiber, and an elongation at break of 55%.

COMPARATIVE EXAMPLE 5

[0091] The hollow fiber membrane prepared in EXAMPLE 3 was drawn inwater at 45° C., but broke at many portions. Furthermore, thesuccessfully stretched portions showed leakage of matter that shouldhave been collected.

COMPARATIVE EXAMPLE 6

[0092] The hollow fiber membrane prepared in EXAMPLE 3 was drawn to 2.5times in polyethylene glycol (molecular weight: 400) at 150° C. Thestretched hollow fiber membrane had a low permeability of 0.5 m³/m²×hrat a differential pressure of 100 kPa and 25° C. because of deformationcaused by melting of the micropores.

COMPARATIVE EXAMPLE 7

[0093] The hollow fiber membrane prepared in EXAMPLE 3 was drawn to 5.5times in water at 85° C. The stretched hollow fiber membrane had anouter diameter of 1.05 mm, an inner diameter of 0.65 mm, a permeabilityof 0.8 m³/m²×hr at a differential pressure of 100 kPa and 25° C., atensile strength of 1,860 g/fiber, and an elongation at break of 32%.The permeability was low because the micropores had a small diameter.

EXAMPLES 12 to 19 AND COMPARATIVE EXAMPLES 8 to 12

[0094] Hollow fiber membranes were prepared as in EXAMPLE 1 except thatγ-butyrolactone was used as the poor solvent and the preparationconditions were varied. The results are shown in Table 2.

[0095] In COMPARATIVE EXAMPLE 8, no hollow fiber membrane was formedbecause of significantly low viscosity of the polymer solutionsdischarged from the spinneret.

[0096] In COMPARATIVE EXAMPLE 9, no hollow fiber membrane was formedbecause of significantly high viscosity of the polymer solutions.

[0097] In COMPARATIVE EXAMPLE 10, the resulting hollow fiber membranehad no clear spherical structures and thus did not show highpermeability. The permeability after stretching was at most 0.4 m³/m²·hrat a differential pressure of 100 kPa and 25° C.

[0098] In COMPARATIVE EXAMPLE 11, the polymer was dissolved for 12 hoursas in COMPARATIVE EXAMPLE 4, but no uniform solution was obtained. Thissolution was gelated when placed into a hopper of the spinning machine,and no hollow fiber membrane was obtained.

[0099] In COMPARATIVE EXAMPLE 12, the resulting hollow fiber membranedid not show permeability and a dense layer was observed on the outerface. TABLE 2 Cooling Cooling Hollow Permeability M.W. of PolymerDissolving Spinneret bath bath section- Outer Inner (m³/m² · hr) TensileElongation polymer conc. temp. temp. temp. conc. forming liquid diameterdiameter at 100 KPa, strength at break (×10⁴) (wt %) (° C.) (° C.) (°C.) (wt %) conc. (wt %) (mm) (mm) 25° C. (g/fiber) (%) EX. 12 41.7 40170 100 27 80 100 1.28 0.78 0.48 400 60 EX. 13 41.7 45 170 120 27 80 1001.22 0.74 0.68 480 59 EX. 14 41.7 38 170  95 28 80 100 1.33 0.99 1.36510 243 EX. 15 41.7 43 170 110 28 80 100 1.49 1.15 8.22 380 70 EX. 1641.7 50 170 140 27 80 100 1.86 1.14 1.40 620 80 EX. 17 41.7 38 170 1008.5 80 100 1.40 0.85 0.89 760 63 EX. 18 35.8 50 170 113 27 80 100 1.340.93 1.07 890 75 EX. 19 41.7 38 150 100 5.5 85  85 1.35 0.85 0.41 1,020200 CE. 8 41.7 18  90  80 — —  90 — — — — — CE. 9 41.7 65 170 — — — — —— — — — CE. 10 41.7 40 185 180 30 80 100 — — — — — CE. 11 28.4 20  78 —— — — — — — — — CE. 12 41.7 40 170 100 27 55 100 — — 0 — —

EXAMPLE 20

[0100] The hollow fiber membrane prepared in EXAMPLE 12 was drawn to 2.2times in water at 80° C. The stretched hollow fiber membrane had anouter diameter of 1.07 mm, an inner diameter of 0.64 mm, a permeabilityof 1.7 m³/m²×hr at a differential pressure of 100 kPa and 25° C., atensile strength of 520 g/fiber, and an elongation at break of 46%.

EXAMPLE 21

[0101] The hollow fiber membrane prepared in EXAMPLE 13 was drawn to 1.6times in water at 80° C. The stretched hollow fiber membrane had anouter diameter of 1.16 mm, an inner diameter of 0.68 mm, a permeabilityof 3.4 m³/m²×hr at a differential pressure of 100 kPa and 25° C., atensile strength of 690 g/fiber, and an elongation at break of 41%.

EXAMPLE 22

[0102] The hollow fiber membrane prepared in EXAMPLE 14 was drawn to 1.7times in water at 81° C. The stretched hollow fiber membrane had anouter diameter of 1.13 mm, an inner diameter of 0.81 mm, a permeabilityof 1.7 m³/m²×hr at a differential pressure of 100 kPa and 25° C., atensile strength of 730 g/fiber, and an elongation at break of 189%.

EXAMPLE 23

[0103] The hollow fiber membrane prepared in EXAMPLE 15 was drawn to 1.5times in water. at 80° C. The stretched hollow fiber membrane had anouter diameter of 1.43 mm, an inner diameter of 1.07 mm, a permeabilityof 10.0 m³m²×hr at a differential pressure of 100 kPa and 25° C., atensile strength of 520 g/fiber, and an elongation at break of 46%.

EXAMPLE 24

[0104] The hollow fiber membrane prepared in EXAMPLE 16 was drawn to 1.9times in water at 87° C. The stretched hollow fiber membrane had anouter diameter of 1.49 mm, an inner diameter of 0.93 mm, a permeabilityof 2.7 m³/m²×hr at a differential pressure of 100 kPa and 250 C, atensile strength of 820 g/fiber, and an elongation at break of 56%.

EXAMPLE 25

[0105] The hollow fiber membrane prepared in EXAMPLE 19 was drawn to 1.5times in water at 87° C. The stretched hollow fiber membrane had anouter diameter of 1.31 mm, an inner diameter of 0.79 mm, a permeabilityof 2.6 m³/m²×hr at a differential pressure of 100 kPa and 25° C., atensile strength of 1,020 g/fiber, and an elongation at break of 130%.

COMPARATIVE EXAMPLE 13

[0106] The hollow fiber membrane prepared in EXAMPLE 12 was drawn inwater at 45° C. The membrane broke at many portions during the drawingstep. The successfully stretched portions of the hollow fiber membranedid not show high permeability.

COMPARATIVE EXAMPLE 14

[0107] The hollow fiber membrane prepared in EXAMPLE 12 was drawn to 3.0times in polyethylene glycol (molecular weight: 400) at 150° C. Thestretched hollow fiber membrane had a low permeability of 0.3 m³/m²×hrat a differential pressure of 100 kPa and 25° C. because of deformationcaused by melting of the micropores.

COMPARATIVE EXAMPLE 15

[0108] The hollow fiber membrane prepared in EXAMPLE 12 was drawn to 5.5times in water at 85° C. The stretched hollow fiber membrane had anouter diameter of 1.00 mm, an inner diameter of 0.60 mm, a permeabilityof 0.29 m³/m²×hr at a differential pressure of 100 kPa and 25° C., atensile strength of 1,560 g/fiber, and an elongation at break of 29%.The permeability was low because the micropores had a small diameter.

EXAMPLE 26

[0109] Into 60 percent by weight of isophorone, 40 percent by weight ofpolyvinylidene fluoride homopolymer having a weight average molecularweight of 417,000 was dissolved at 155° C. The polymer solution wasdischarged from a spinneret at 100° C. while 100% isophorone (hollowsection-forming liquid) was being discharged into the hollow section ofthe inner tube of the spinneret, and was solidified in a cooling bathcontaining 80 percent by weight of isophorone at 30° C. The fiber wasdrawn to 3.0 times in water at 80° C. The stretched hollow fibermembrane had an outer diameter of 1.40 mm, an inner diameter of 0.90 mm,a permeability of 2.8 m³/m²×hr at a differential pressure of 100 kPa and25° C., a tensile strength of 1,010 g/fiber, and an elongation at breakof 54%.

EXAMPLE 27

[0110] Into 60 percent by weight of dimethyl phthalate, 40 percent byweight of polyvinylidene fluoride homopolymer having a weight averagemolecular weight of 417,000 was dissolved at 165° C. The polymersolution was discharged from a spinneret at 110° C. while a hollowsection-forming liquid of 60 percent by weight of dimethyl phthalate and40 percent by weight of ethylene glycol (molecular weight: 400) wasbeing discharged into the hollow section of the inner tube of thespinneret, and was solidified in a cooling bath containing 60 percent byweight of dimethyl phthalate and 40 percent by weight of ethylene glycol(molecular weight: 400) at 40° C. The fiber was drawn to 3.0 times inethylene glycol (molecular weight: 400) at 120° C. The stretched hollowfiber membrane had an outer diameter of 1.35 mm, an inner diameter of0.75 mm, a permeability of 1.8 m³/m²×hr at a differential pressure of100 kPa and 25° C., a tensile strength of 1,410 g/fiber, and anelongation at break of 38%.

EXAMPLE 28

[0111] Into 60 percent by weight of γ-butyrolactone, 40 percent byweight of polyvinylidene fluoride homopolymer having a weight averagemolecular weight of 417,000 was dissolved at 150° C. to prepare ahomogeneous solution. The solution had a crystallization temperature Tcof 57° C. Thus, the preferable discharge temperature lies within therange 57° C.≦Ts≦147° C. The polymer solution was allowed to stand at110° C. for defoaming and was discharged at 100° C. (spinnerettemperature Ts) from the outer pipe of a double pipe spinneret, while100 percent by weight of γ-butyrolactone was being supplied into thehollow section from the inner tube of the double pipe spinneret. Thesolution was discharged into a cooling bath at 5° C. having a distancebetween the spinneret surface and the cooling bath surface of 4 cm andcontaining 80 percent by weight of γ-butyrolactone and 20 percent byweight of water at an extruding rate of 6.0 m/min and an average coolingrate of 14,250° C./min to allow the solution to gelate in the coolingbath. The resultant extrudate was drawn to 1.5 times in a hot water bathat 80° C. to prepare a hollow fiber membrane.

[0112] The properties of the hollow fiber membrane are shown in Table 4.The hollow fiber membrane was excellent in mechanical strength,permeability, and separability. The membrane had a structure ofintegrated spherulites having a particle size of 1.8 μm with poresextending between the spherulites.

EXAMPLE 29

[0113] Into 67 percent by weight of γ-butyrolactone, 33 percent byweight of polyvinylidene fluoride homopolymer having a weight averagemolecular weight of 417,000 was dissolved at 120° C. to prepare ahomogeneous solution. The solution had a crystallization temperature Tcof 41° C. Thus, the preferable discharge temperature lies within therange 41° C.≦Ts≦131° C. A hollow fiber membrane was prepared as inEXAMPLE 1 according to conditions shown in Table 3. A liquid mixture of90 percent by weight of γ-butyrolactone and 10 percent by weight ofwater was supplied into the hollow section. The properties of the hollowfiber membrane are shown in Table 4. The hollow fiber membrane wasexcellent in permeability and separability. The membrane had a structureof integrated spherulites having a particle size of 3.2 μm with poresextending between the spherulites.

EXAMPLE 30

[0114] Into 45 percent by weight of propylene carbonate, 55 percent byweight of polyvinylidene fluoride homopolymer having a weight averagemolecular weight (Mw) of 358,000 was dissolved at 170° C. to prepare ahomogeneous solution. The solution had a crystallization temperature Tcof 78° C. Thus, the preferable discharge temperature lies within therange 78° C.≦Ts≦168° C. A hollow fiber membrane was prepared as inEXAMPLE 1 according to conditions shown in Table 3, wherein propylenecarbonate was supplied into the hollow section and was also used in thecooling bath. The properties of the hollow fiber membrane are shown inTable 4. The hollow fiber membrane was excellent in mechanical strength,permeability, and separability. The membrane had a structure ofintegrated spherulites having a particle size of 1.9 μm with poresextending between the spherulites.

EXAMPLE 31

[0115] Into 45 percent by weight of propylene carbonate, 55 percent byweight of polyvinylidene fluoride homopolymer having a weight averagemolecular weight (Mw) of 417,000 was dissolved at 170° C. to prepare ahomogeneous solution. The solution had a crystallization temperature Tcof 79° C. Thus, the preferable discharge temperature lies within therange 79° C.≦Ts≦179° C. A hollow fiber membrane was prepared as inEXAMPLE 3 according to conditions shown in Table 3. The thermographicresults showed that the hollow fiber was cooled to 79° C. or less at aposition 3 cm below the spinneret, and the calculated average coolingrate Vt in the membrane-forming step according to method (a) was 3,500°C./min.

[0116] The properties of the hollow fiber membrane are shown in Table 4.The hollow fiber membrane was excellent in permeability andseparability. The membrane had a structure of integrated spheruliteshaving a particle size of 2.2 μm with pores extending between thespherulites.

[0117] COMPARATIVE EXAMPLE 16

[0118] Into 65 percent by weight of γ-butyrolactone, 35 percent byweight of polyvinylidene fluoride homopolymer having a weight averagemolecular weight of 444,000 was dissolved at 130° C. to prepare ahomogeneous solution. The solution had a crystallization temperature Tcof 47° C. Thus, the preferable discharge temperature lies within therange 47° C.≦Ts≦137° C. A hollow fiber membrane was prepared as inCOMPARATIVE EXAMPLE 28 according to conditions shown in Table 3. Theresults are shown in Table 4. The hollow fiber membrane showed a smallrejection of 33% to uniform polystyrene latex particles having adiameter of 0.309 μm. The membrane had a structure of integratedspherulites having a particle size of 4.3 μm with pores extendingbetween the spherulites. A small average cooling rate is assumed toincrease the particle size of the spherulites and thus to decrease thefilterability due to an increased pore size.

COMPARATIVE EXAMPLE 17

[0119] Discharge of the polymer solution prepared in EXAMPLE 28 wastried at a spinneret temperature Ts of 50° C. which was below thecrystallization temperature Tc; however, the solution could not bedischarged because of solidification of the polymer in the spinneret.

[0120] COMPARATIVE EXAMPLE 18

[0121] A hollow fiber membrane was prepared as in EXAMPLE 28 except thatthe spinneret temperature Ts was 150° C. The properties of the hollowfiber membrane are shown in Table. 4. The hollow fiber membrane showed asmall rejection of 44% to uniform polystyrene latex particles having adiameter of 0.309 μm. The membrane had a structure of integratedspherulites having a particle size of 5.1 μm with pores extendingbetween the spherulites.

COMPARATIVE EXAMPLE 19

[0122] Into 75 percent by weight of γ-butyrolactone, 25 percent byweight of polyvinylidene fluoride homopolymer having a weight averagemolecular weight of 444,000 was dissolved at 130° C. to prepare ahomogeneous solution. The solution had a low crystallization temperatureTc of 31° C. A hollow fiber membrane was prepared as in COMPARATIVEEXAMPLE 28 according to conditions shown in Table 3. The properties ofthe hollow fiber membrane are shown in Table 4. The hollow fibermembrane showed a small rejection of 40% to uniform polystyrene latexparticles having a diameter of 0.309 μm. The membrane had a structure ofintegrated spherulites having a particle size of 4.3 μm with poresextending between the spherulites.

COMPARATIVE EXAMPLE 20

[0123] Into 78 percent by weight of γ-butyrolactone, 22 percent byweight of polyvinylidene fluoride homopolymer having a weight averagemolecular weight of 444,000 was dissolved at 145° C. to prepare ahomogeneous solution. The solution had a high crystallizationtemperature Tc of 121° C. The polymer solution was allowed to stand at145° C. for defoaming. A hollow fiber membrane was prepared as inCOMPARATIVE EXAMPLE 28 according to conditions shown in Table 3. Theproperties of the hollow fiber membrane are shown in Table 4. The hollowfiber membrane showed no water permeability (0 m³/m²×hr at adifferential pressure of 100 kPa and 25° C.). TABLE 3 Polymer solutionPoor solvent Cooling bath Dry Extruding Average Polymer Tc content ininjected Poor solvent Temp. Ts distance rate cooling temp. Drawing conc.(wt %) (° C.) liquid (wt %) content (wt %) (° C.) (° C.) (cm) (m/min) (°C.) (times) EX. 28 40 57 100 80 5 100 4 6.0 14,250 1.5 EX. 29 33 41  9080 30 100 5 7.0 7,000 Not drawn EX. 30 55 78 100 85 5 160 0.5 15.046,500 1.8 EX. 31 55 79 100 70 15 100 30 5.0 3,500 1.5 CE. 16 35 47 10080 40  80 10 4.0 1,600 1.5 CE. 17 40 57 100 80 5  50 — — — — CE. 18 4057 100 80 5 150 4 6.0 21,750 1.5 CE. 19 25 31 100 80 5  80 4 6.0 11,2501.5 CE. 20 78 121  100 80 40 145 4 6.0 15,750 1.5

[0124] TABLE 4 Inner diameter Outer diameter Tensile strength Elongationat Permeability Rejection to (mm) (mm) (g/fiber) break (%) (m³/m² · h ·kPa) polystyrene latex (%) EXAMPLE 28 0.82 1.35 1,780 85 3.1 85 EXAMPLE29 0.75 1.20 — — 4.5 78 EXAMPLE 30 0.78 1.37 2,060 75 1.9 82 EXAMPLE 310.88 1.45 — — 1.3 95 COMPARATIVE 0.69 1.29 680 72 4.5 33 EXAMPLE 16COMPARATIVE — — — — — — EXAMPLE 17 COMPARATIVE 0.63 1.23 1,150 83 3.8 44EXAMPLE 18 COMPARATIVE 0.69 1.29 680 72 4.2 40 EXAMPLE 19 COMPARATIVE0.73 1.42 — — 0   — EXAMPLE 20

EXAMPLE 32

[0125] The hollow fiber membrane prepared in EXAMPLE 25 was immersedinto an aqueous 50 weight percent ethanol solution and then into reverseosmosis (RO) water. The hollow fiber membrane was allowed to stand in anaqueous 0.01-N sodium hydroxide solution at 30° C. for 1 hour, then waswashed with RO water. The membrane was allowed to stand in an aqueous1.5 weight percent hydrogen peroxide solution at 30° C. for 1 hour, andwas washed with RO water.

[0126] The water permeability of the treated hollow fiber membraneincreased to 3.2 m³/m³×hr at a differential pressure of 100 kPa and 25°C. The tensile strength was 1,100 g/fiber and the elongation at breakwas 125%.

EXAMPLE 33

[0127] The hollow fiber membrane prepared in EXAMPLE 25 was immersedinto an aqueous 50 weight percent ethanol solution and then into ROwater. The hollow fiber membrane was allowed to stand in an aqueous0.01-N sodium hydroxide solution at 30° C. for 1 hour, then was washedwith RO water. The membrane was allowed to stand in water containing10-ppm ozone 100 hour. The water permeability of the treated hollowfiber membrane increased to 3.5 m³/m²×hr at a differential pressure of100 kPa and 25° C. The tensile strength was 1,000 g/fiber and theelongation at break was 110%.

EXAMPLE 34

[0128] The hollow fiber membrane prepared in EXAMPLE 25 was immersedinto an 10 weight percent N-N,-dimethyl-1,3-propanediamine in ethanolsolution at 30° C. for 1 hour. The water permeability of the treatedhollow fiber membrane significantly increased to 4.1 m³/m²×hr at adifferential pressure of 100 kPa and 25° C. The tensile strength was1;300 g/fiber and the elongation at break was 75%.

EXAMPLE 35

[0129] Using a pressured hollow fiber microfiltration membrane thatincluded bundled hollow fiber membranes prepared in EXAMPLE 32 and had alength of about 50 cm and an effective membrane area of 0.5 mm², LakeBiwa water was filtered by dead end filtration at a constant flow rate.FIG. 5 is a schematic diagram of a membrane separation apparatus used.Lake Biwa water during the filtration operation had an average turbidityof 6.8 NTU and an average TOC of 2.3 mg/l.

[0130] In the filtration operation, raw water was supplied into aseparation membrane module 3 by a pressurizing pump 2, then anelectromagnetic valve 6 a was closed to reserve the raw water in atreated water reservoir 5. The membrane permeation rate (F) was set tobe 2 m³/m²·d. For physical washing, 1-minute back washing and 1-minuteair scrubbing were performed for 1 minute for every 30-minute operation.In the back washing, back washing water that was supplied from thetreated water reservoir 7 through an electromagnetic valve 6 e wasallowed to flow from the treated water face to the raw water face of theseparation membrane module 3 through an electromagnetic valve 6 d and toflow out through the electromagnetic valve 6 a. A sodium hypochloritesolution at a concentration of 5 mg/l (0.07 times the average TOC of theraw water) was added to the back washing water. In the air scrubbingwashing, air was introduced from the bottom of the separation membranemodule 3 to vibrate the hollow fiber membrane. After this operation, anelectromagnetic valve 6 c was opened to drain dirty water in theseparation membrane module 3. An electromagnetic valve 6 b was closedduring the physical washing operation. The filtration differentialpressure after 1,000-hour operation was about 60 kPa, which was a lowlevel.

COMPARATIVE EXAMPLE 21

[0131] A filtration operation was performed at the same time as inEXAMPLE 35 except that the hollow fiber membrane prepared in EXAMPLE 25was used. The filtration differential pressure after 1,000 -houroperation was about 95 kPa, which was higher than that in EXAMPLE 35 andwas a disadvantageous level in view of operation stability and cost.

COMPARATIVE EXAMPLE 22

[0132] A filtration operation was performed at the same time as inEXAMPLE 35 except that no sodium hypochlorite was added. The filtrationdifferential pressure reached 100 kPa in a day, and the apparatus wasnot able to continue the operation.

What is claimed is:
 1. A method of producing a hollow fiber membranecomprising: discharging a polyvinylidene fluoride solution comprising apolyvinylidene fluoride resin and a poor solvent at a temperature abovea phase separation temperature into a cooling bath at a temperaturebelow the phase separation temperature to coagulate the polyvinylidenefluoride resin.
 2. The method according to claim 1, wherein thecrystallization temperature of the polyvinylidene fluoride solution isin the range of 40° C. to 120° C., the average cooling rate of thepolyvinylidene fluoride solution when the temperature of thepolyvinylidene fluoride solution reaches the crystallization temperatureduring cooling is in the range of 2×10³° C./min to 10⁶° C./min, and thetemperature Ts of a spinneret for discharging the polyvinylidenefluoride solution into the cooling bath and the crystallizationtemperature Tc satisfy the relationship Tc≦Ts≦Tc+90.
 3. The methodaccording to claim 1, wherein the polyvinylidene fluoride solutioncontains at least 20 to 60 percent by weight of the polyvinylidenefluoride resin.
 4. The method according to claim 1, wherein thepolyvinylidene fluoride solution has a phase separation temperature inthe range of 80° C. to 220° C.
 5. The method according to claim 1,wherein the cooling bath contains 60 to 100 percent by weight of a poorsolvent.
 6. The method according to claim 1, wherein the hollow sectionof the hollow fiber membrane is formed by using a hollow section-formingliquid containing 60 to 100 percent by weight of a poor solvent.
 7. Themethod according to claim 1, wherein the coagulated polyvinylidenefluoride resin is drawn to 1.1 to 5 times at a temperature in the rangeof 50° C. to 140° C.
 8. A hollow fiber membrane comprising apolyvinylidene fluoride resin having spherical structures that have anaverage diameter in the range of 0.3 to 30 μm.
 9. The hollow fibermembrane according to claim 8, having spherical structures at a densityin the range of 10³/mm² to 10⁸/mm² in the interior.
 10. The hollow fibermembrane according to claim 8, having micropores with11 an average poresize in the range of 0.01 to 20 μm in the outer surface.
 11. The hollowfiber membrane according to claim 8, having a porosity in the range of40% to 75%.
 12. The hollow fiber membrane according to claim 8, having awater permeability in the range of 0.1 to 10 m³/m²·hr at 100 kPa and 25°C., a tensile strength in the range of 0.3 to 3 kg per fiber, and anelongation at break in the range of 20% to 1,000%.
 13. The hollow fibermembrane according to claim 8, wherein the main chain of thepolyvinylidene fluoride has hydrophilic functional groups.
 14. Thehollow fiber membrane according to claim 13, wherein the hydrophilicfunctional group is at least one selected from hydroxyl, amino, andcarboxyl groups.
 15. A hollow fiber membrane module comprising thehollow fiber membrane according to claim
 8. 16. A water separatorcomprising the hollow fiber membrane module according to claim
 15. 17. Amethod of producing permeated water from raw water using the waterseparator according to claim
 16. 18. A method of producing permeatedwater from raw water using a membrane comprising a polyvinylidenefluoride resin, the method comprising bringing the membrane into contactwith chlorine in an amount corresponding to the organic content in theraw water.
 19. The method according to claim 17, wherein the membrane isbrought into contact with chlorine in an amount corresponding to theorganic content in the raw water.
 20. The method according to eitherclaim 18 or 19, wherein the amount of the chlorine is 0.01 to 10 timesthe total organic carbon content in the raw water.